Blog comments are visible and open to anyone reading the blog post. And the blog author can always see them and reply to them instead of chasing discourse across the internet.
Reddit and HN topics become read-only after some time, so I'm not sure how they can replace blog comments.
Twitter for comments, I don't understand. I guess you link to your canonical tweet and people reply to it? I find threaded conversation on Twitter to be just about unbearable.
That being said, there are some decent Open Source Disqus clones out there you could probably use!